Air Conditioners and 1st law

  • Thread starter shetland
  • Start date
  • #1
shetland
17
0
Can you use the heat energy extracted by an A/C to do some useful work? Or does this somehow violate the 1st law in such a way to negate energy conservation...doesn't appear so; of course, the heat energy coming out into the ambient temp would be small, nil, and thus the kind of work you could get out would be poor quality.

shelley
 

Answers and Replies

  • #2
zoobyshoe
6,551
1,287
There is nothing to prevent you
from using the heat extracted by
an AC unit to do work. The ideal
power available will be in
proportion to the difference in
temperature between the heat ex-
tracted and the ambient temp of
the environment you put it to
work in. In the average case this
will hardly be nil. There would
be enough energy to make practical
use of if you're not too ambit-
ious.
 
  • #3
Bystander
Science Advisor
Homework Helper
Gold Member
5,406
1,493
Yes. No. If you want to "drive" a thermopile with the temperature difference established by the A/C, there's no problem --- do you accomplish anything? No.
 
  • #4
shetland
17
0
Thanks, Zoobyshoe (grt handle, btw).

Ok, maybe I shouldn't have said nil, but I think the efficiency of this type of heat engine is less than for instance in the case for internal combustion.

So, the problem would come down to collecting (maybe not so big, since most A/C's have nice heat exhcanger fins), and then with the relatively low temp, what could you do with it?

I mean, if this was a relatively simple thing, wouldn't we have expected to see this as more standard equipment on a/c's for industry? (since they use more kilowatts for their pumps), or even on homes?

One reason I thought about this, is we are considering adding solar power to our home, and it was pointed out, given our location - sunny southern ca, and if we have enough panels, that the net input at times *could* possibly be greater than consumption, and that this extra amt. can be fed back into the grid, reducing our energy costs. I was simply wondering if there was a similar way to reduce the cost of a/c this way, by making use of the wasted thermal energy...
 
  • #5
Dissident Dan
237
2
Wow, it's funny that you should mention this. I had totally fogotten about that, but some people from the Physical Plant at my university gave a presentation about energy stuff.

One of the guys talked about how he used the heat from his air conditioning. It was used to heat water. It had a very significant impact on the amount of electricity that he had to purchase to heat water.

Someone mentioned that it probably wasn't feasable, based on the evidence that it isn't something that you see. The sad truth is that once a technology becomes well-established, change tends to become very unlikely. I wouldn't doubt that we live in the most energy-inefficient country in the world.
 
  • #6
zoobyshoe
6,551
1,287
In practical terms the best use
that might be made of the heat
from an AC unit would be to rig
it up to work in conjunction with
your water heater, to heat, or at
least preheat water from the cold
water mains.

This would best be accomplished at
the manufacturing level with all
the proper engineering. A person
would have to be a serious tinker-
er to spend the time and energy to
retrofit an AC unit to do this.
There is the "contraption" factor
to consider: would it end up being
something that detracted from the
appearance of your home?
The energy, however, is there and
available.

Since you live here in Sunny So.
Cal. it would actually be easier
to install solar water heating
panels to cut down on your water
heating bill. This would require
plumbing them into your water
heater, (and might require permits
for all I know). It is something
you'd have to research to see if
it is worth it in the long run.

-Zoobyshoe
 
  • #7
zoobyshoe
6,551
1,287
Whoa! Dissident Dan! You posted
while I was composing mine! Spooky
mental wavelength matchup!
 
  • #8
shetland
17
0
Zooby,

I was more curious about this from the standpoint of thermodynamics, and assure you, I'm not nearly nutty enough to try this at home!

I'm taking a biophysics class at the moment, and we just revisited gibbs free energy and the role of 'free energy transducers', such that living organisms apparnelty play..got me to thinkin 'bout thermo again (took it many, many years ago as an undergraduate..and I'm RUSTY!
 
  • #9
Dissident Dan
237
2
Originally posted by zoobyshoe
"Spooky mental wavelength matchup!", Batman!
 
  • #10
zoobyshoe
6,551
1,287
Originally posted by shetland
I was more curious about this from the standpoint of thermodynamics...
In this case let me just say that
there is probably more energy in
this "waste" heat than you susp-
ected at first, and, as I said,
if you not too ambitious, it
could be put to some practical
use.

-Zoob
 
  • #11
zoobyshoe
6,551
1,287
Originally posted by Dissident Dan
"Spooky mental wavelength matchup!",Batman!
Agreed, Robin. Could this be the
work of that dastardly fiend, Fz+
and his Neo-Quantum thought inser-
tion ray gun? To the Batmobile!
 
  • #12
frankR
91
0
Interesting idea. I imagine the only thing you could do with the heat is heat water. Some how incorporate that heated water into a system with the water heater. Surprisingly that sounds like a practical idea, could save quite a bit of energy over a few months of use during the summer. But if you're truly conscious about conserving energy, why not just use solar power. ;)
 
  • #13
zoobyshoe
6,551
1,287
Originally posted by frankR
Interesting idea. I imagine the only thing you could do with the heat is heat water.
I bet you could roast a Turkey.
 
  • #14
russ_watters
Mentor
21,950
8,984
You guys are treating this idea well, but this sort of thing is where a lot of free energy claims come from. People often make the mistake of thinking they can recover enough waste heat from a cycle to run the cycle - a direct violation of the 1st law. Thats what a self-powered water wheel does.

Yeah, you could use your A/C to heat water but I'm not sure how practical it would be. Probably wouldn't do much good.
 
  • #15
selfAdjoint
Staff Emeritus
Gold Member
Dearly Missed
6,881
10
The only limitation from thermodynamics is that you couldn't recover enough energy from the waste heat to run the airconditioner that produced it. That would be perpetual motion.

The trouble with using it is that devices that work off small heat differences tend to be huge and slow. As the folks who worked with those old solar hot water systems found.
 
  • #16
Artman
1,509
31
What this thread is talking about is exactly what a water source heat pump loop is designed to do. The explanation is a little long, but here goes.

A water source heat pump is basically an Air Conditioning unit with a fan, a direct expansion coil, a compressor a condensor heat exchanger and a reversing valve to change the direction of refrigerant from evaporator coil to condensing heat exchanger when shifting from cooling to heating. Heat pumps cool in the warm weather and heat in the cold. The water source loop takes the rejected heat from the unit and cools the refrigerant.

With that in mind, imagine a building with a lot of outside walls and a bunch of inside rooms with a lot of heat load like computers. The heap pimps on the outside of the building might have to heat in the morning because of the cold air outside so they pull heat from the water loop. At the same time the rooms on the inside of the building may be hot because of the computers putting heat into the room. The heap pump takes that heat out of the air and adds the heat to the water loop raising the loop temperature back up.

In theory, this kind of system balances well in the cold weather and no heat has to be added to the water or removed. In the warm weather the whole building is hot and needs to be cooled so the water loop temperature rises and must be rejected to the air through a cooling tower or to another heat sink such as the ground in geothermal or the water in a water to water heat exchanger. But in the cold weather, there is a savings of energy because they don't have to heat the water loop as it self balances.
 
  • #17
Dissident Dan
237
2
Originally posted by frankR
. But if you're truly conscious about conserving energy, why not just use solar power. ;)

Well, you usually can't get all the heat that most USAmericans desire just from a passive solar system, unless maybe you had a really large tank. You could use this in conjunction with the passive solar.

Also, with this system, you could keep all the working behind walls, which may have an aesthetic component that some people desire.
 
  • #18
Artman
1,509
31
I forgot another related item oftem placed in homes with geothermal heat pumps is the preheater for domestic hot water. This uses a heat exchanger to preheat the water going to the water heater and furhter cool the water going to the AC units. Even if it can't cover the heater load, it can contribute and also reduce the thermal shock on the water heater.
 
  • #19
krab
Science Advisor
893
3
You need the 2nd law to understand what can be done with the heat energy. It tells you that if you want to use the heat rejected by an a/c unit (and that unit is already reasonably efficient), the unit will then be rejecting its heat into a reservoir of slightly higher temperature. This requires the unit to use more energy. This extra energy will always be greater than the mechanical energy you can get out of the rejected heat.

In symbols, let's say the a/c unit is cooling the reservoir at temp. T1, and dumping heat into ambient outside temp. T2. It is also applying work W1. To use some of this heat, the a/c unit will effectively be dumping heat into a reservoir of temp. T3, where T3>T2. To do this, W1 increases to W2. Now you can run a heat engine between T3 and T2 to produce work W3. But always W3<W2-W1, so nothing is gained.
 
  • #20
zoobyshoe
6,551
1,287
the unit will then be rejecting its heat into a reservoir of slightly higher temperature.
Where do you get this?
 
  • #21
Ivan Seeking
Staff Emeritus
Science Advisor
Gold Member
8,010
1,010
Originally posted by zoobyshoe
the unit will then be rejecting its heat into a reservoir of slightly higher temperature.
Where do you get this?

This would normally be true since the exit temp of the heat exchanger [A/C condenser to water preheater] would be well above the ambient temp.
 
  • #22
zoobyshoe
6,551
1,287
In order to expell heat into the
ambient temperature the AC unit
it required to concentrate
the heat to a temperature higher
than the ambient temperture via
work. It is going to do this
whether or not you use this higher
than ambient temperature heat to
do something else.

If instead of letting it flow into
the ambient atmosphere you expose
it to water coming cold from the
main the heat will flow into that
water and you have gained hotter
water. This does not require more
work from the compressor.

I do not see where this notion of
trying to put heat into a reser-
voir of slightly higher temperat-
ure came from.
 
  • #23
Artman
1,509
31
Originally posted by krab
...if you want to use the heat rejected by an a/c unit (and that unit is already reasonably efficient), the unit will then be rejecting its heat into a reservoir of slightly higher temperature. This requires the unit to use more energy. This extra energy will always be greater than the mechanical energy you can get out of the rejected heat...

Not if other units are sharing the reservoir and removing the excess heat for heating operations elsewhere, or if the system is storing the heat for future use.

Here is a quote from the Trane Applications Engineering Manual for Water Source Heat Pump System Design. Pg 17. "Energy cost savings can be accrued by recovering heat from interior zones for use near the building perimeter."

Also, if you throw in a storage tank to store large quantities of low temperature water in the evenings you can bleed it into the system during the cooling operation of the entire building, then during the cooling operation heat is placed into the tank to offset early evening heating loads. The idea is to store enough to meet the high demand periods instead of heat being added by a boiler or removed from the system by a cooling tower.

This is common practice and not in anyway contrary to the laws of thermodynamics.
 
  • #24
marcus
Science Advisor
Gold Member
Dearly Missed
24,770
791
Whoah! I see now that krab (7 September 10:25 PM post) already covered the thermodynamic formula business. Had not noticed krab's when I wrote this:

...does anyone want to comment on a simple formula from thermodynamics that limits the efficiency of AC

the work (or electricity) cost of pumping a joule of heat from Lo temp to Hi temp
is always going to be at least

(H-L)/L joules of work (or electricity)

in the metric system they prefer for you to measure heat in joules instead of calories or BTU or kWh-----whatever you measure electricity in should also be your measure of heat, that way there are no conversions

but you could also be more userfriendly and say

"the work or electricity cost of pumping a kWh of heat from Lo temp to Hi temp is always going to be at least

(H-L)/L kWh of work or electricity"

what units doesn't matter as long as consistent.

So if you keep the inside temp 290 and the outside temp is 310
then
(H-L)/L = (310 - 290)/290 = 20/290

and that is the unavoidable cost in kWh of pumping a kWh of heat from your livingroom and dumping it outdoors

So this makes it clear what you have to play around with

If you have a flow of water from the city water main which you are using anyway for something or an underground tank of water or whatever that is cooler than ambient then you can reduce H.

Since you reduce H you will reduce (H - L)/L which is the energy cost.

If your AC is efficient it does not seem to matter whether you save energy by reducing the cost of running it by reducing H to H'
or whether you continue dumping AC heat at H and run a little engine on the side between H and H' to generate some back.

If you have a flow of cold water to dump heat into (say) then you get the same energy bonus whether you use it to cool the cooling vanes of the AC directly or use it to run an engine that generates power on the temp difference between the vanes of the AC and the cold water heatsink.

Improving efficiency of the basic unit and using less can be like having an aux generator cranking power on the side. And may be less costly in terms of capital equipment
 
Last edited:
  • #25
krab
Science Advisor
893
3
Originally posted by zoobyshoe
In order to expell heat into the
ambient temperature the AC unit
it required to concentrate
the heat to a temperature higher
than the ambient temperture via
work. It is going to do this
whether or not you use this higher
than ambient temperature heat to
do something else.

Quite right. The rate of heat transfer to the hotter reservoir (usu. the outside air at temp. T2) is proportional to the temperature difference T3-T2, where T3 is the temp. of the working fluid as it comes into the radiator. The a/c unit is working effectively between temps T3 and T1 and so is less efficient than if it were working between T2 and T1. In place of the radiator, you can set up a heat engine working between T3 and T2. But a better scheme would be to reduce T3, and thereby save mechanical energy by making the a/c more efficient. How? Since you still need to radiate the same amount of heat, a lower T3 would require a larger radiator. It's kinda counter-intuitive, I know.

Originally posted by zoobyshoe
If instead of letting it flow into
the ambient atmosphere you expose
it to water coming cold from the
main the heat will flow into that
water and you have gained hotter
water. This does not require more
work from the compressor.

That's true. But if you already have cold water from the main to cool the air, you don't need an air conditioner; you just need a heat exchanger. Compared to this arrangement, which reqires zero work from compressor, you do need more work from compressor.

To avoid misunderstanding, let me emphasize I'm talking about what can be done theoretically. You are talking about having a device of fixed characteristics. In that case, of course you can recover some of the wasted heat energy and put it to use. My point is that energy recovery would be more trouble than redesigning your commercial unit to make it more efficient.
 
  • #26
Loren Booda
3,119
4
Ultrasonic, ultraefficient cooling

No mention yet of one of my favorite, and overlooked, inventions - using an ultrasonic cavity driven by one moving part, a transducer, to cool (rarify) and heat (compress) air near alternate metallic fins. The overall design is beautifully simple. Its efficiency may be twice that of conventional air conditioners. The waste heat, more thermodynamically "ideal," can be recovered directly. Why aren't they yet on the market?
 
  • #27
zoobyshoe
6,551
1,287
Originally posted by krab
You need the 2nd law to understand what can be done with the heat energy. It tells you that if you want to use the heat rejected by an a/c unit (and that unit is already reasonably efficient), the unit will then be rejecting its heat into a reservoir of slightly higher temperature.
What you have said here is that
the act of using the rejected heat
creates a situation where the unit
is now required to deal with a
higher ambient temp. I do not
understand why you said this. T2
and T3 remain the same whether or
not you harness the difference to
do something else.

This requires the unit to use more energy.
I do not know why you said this.
Using the T3-T2 difference in no
way increases the load on the
unit.

In symbols, let's say the a/c unit is cooling the reservoir at temp. T1, and dumping heat into ambient outside temp. T2. It is also applying work W1. To use some of this heat, the a/c unit will effectively be dumping heat into a reservoir of temp. T3, where T3>T2. To do this, W1 increases to W2.
There is no increase from W1 to
W2 from using T3-T2 to do work.
W1 remains constant and will
happen whether or not you use T3-
T2. The compressor is aleady
required to achieve T3 in
order for the heat removed from
the building to flow "downhill" as
per the second law into the air
outside. Using this "flow" does
not cause the compressor to have
to increase to W2.
 
  • #28
zoobyshoe
6,551
1,287
Originally posted by marcus
...does anyone want to comment on a simple formula from thermodynamics that limits the efficiency of AC

the work (or electricity) cost of pumping a joule of heat from Lo temp to Hi temp
is always going to be at least

(H-L)/L joules of work (or electricity)

in the metric system they prefer for you to measure heat in joules instead of calories or BTU or kWh-----whatever you measure electricity in should also be your measure of heat, that way there are no conversions
What I observe is that it costs
less to go from 310 to 300 than
from 300 to 290. It is not the #
of degrees alone that matters but
also where you start and finish
your temperature drop.
A twenty degree drop costs more
than adding the first ten degree
drop to itself would cost.
Is this the limit in efficiency
you referred to?

-Zooby
 
  • #29
Duncan
9
0
Heating batch water

This discussion remains me of a demostration I saw on TV. I think it was the http://www.open2.net/scienceshack/tvsumm_intro.htm . They showed how by using a fridge(heat pump) and
river water, a bath full of water could be heated for less energy then by directly heating the water. Unfortunately I cannot find a link to the specific episode.
So why don't all houses have a heat pump to heat them?


Duncan
 
  • #30
Artman
1,509
31


Originally posted by Loren Booda
No mention yet of one of my favorite, and overlooked, inventions - using an ultrasonic cavity driven by one moving part, a transducer, to cool (rarify) and heat (compress) air near alternate metallic fins. The overall design is beautifully simple. Its efficiency may be twice that of conventional air conditioners. The waste heat, more thermodynamically "ideal," can be recovered directly. Why aren't they yet on the market?

I think I read that they use this technology for small transportation refrigeration units. Like coolers in SUV's or boats.
 
  • #31
Loren Booda
3,119
4
Artman, what you're referring to, I believe, is a comparatively inefficient electronic solid state device (which however has no moving parts). It puts out a lot of heat, from what I recall. If only it could be improved. Does anyone know the physics behind it?
 
  • #32
russ_watters
Mentor
21,950
8,984
I think that's a "Peltier" device. I have no idea about efficiency, but some people use them to cool computer cpus.
 
  • #33
Artman
1,509
31
Originally posted by russ_watters
I think that's a "Peltier" device. I have no idea about efficiency, but some people use them to cool computer cpus.

Yes that was it. I'm not sure about the efficiency either.
 
  • #34
ray b
176
0
I had a water pre-heater on my central A/C at my last house about 5 years ago
I put it in myself, with the help of an A/C teck to do the connections
it was a regular comercial unit that took the output freon from the compressor and extracted some heat before the freon ran thru the normal cooling finned coils and dumped the heated water into the home water heater
it was a small box, with a small water pump and coils for the water and A/c freon
mounted outside near the A/c unit

the freon hook up part was way eazyer then the water pipes routing under the house to this unit and back to the water heater

saving were minimal and hard to see as the new central A/C was the big incresse in power use but it allso lowered the heat load on the a/c unit
 
  • #35
zoobyshoe
6,551
1,287
ray b,

It would have been nice if you
could have gotten about a years
worth of bills without the water
preheater in use to compare with
a years worth of bills after you
got it hooked up. Then we could
have a more concrete idea of what
you were actually saving by using
the AC unit to preheat water.

When I was thinking about this I
was working off the assumption
there would be a way to use the
water pressure itself to circulate
it through the system. Adding a
pump cuts down on the benefits.

-Zooby
 

Suggested for: Air Conditioners and 1st law

Replies
1
Views
452
Replies
17
Views
987
Replies
1
Views
1K
Replies
12
Views
2K
  • Last Post
Replies
7
Views
857
  • Last Post
Replies
9
Views
1K
  • Last Post
Replies
5
Views
730
Replies
2
Views
575
  • Last Post
Replies
3
Views
831
  • Last Post
Replies
7
Views
9K
Top